Recycling furnace for burning off precious metal-containing materials

ABSTRACT

A recycling furnace is provided for processing potentially explosive precious metal-containing materials having organic fractions that combust with great energy. The furnace includes a switching facility for alternating operation of two burning-off chambers of the furnace between: (A) pyrolysis or carbonization under protective furnace gas in an atmosphere comprising maximally 6 wt-% oxygen, and (B) oxidative combustion of the organic fractions including carbon. The furnace has indirect heating and a control that determines the end of the pyrolysis or carbonization by a sensor and controls the switching facility to supply air or oxygen to the interior of the furnace. A continuous conveyor for dosing of liquids or liquefied substances during the pyrolysis is controlled by at least one parameter of post-combustion, preferably a temperature sensor. A single waste gas treatment facility is used for thermal post-combustion for the two furnace chambers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.12/056,524, filed Mar. 27, 2008, now U.S. Pat. No. 8,188,329, which wasa Continuation of International Application No. PCT/EP2006/009267, filedSep. 25, 2006, which was published in the German language on Apr. 5,2007, under International Publication No. WO 2007/036334 A2 and thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method and corresponding device forburning-off precious metal-containing materials.

The following methods are customary on an industrial scale for theprocessing of precious metal-containing waste materials with relativelyhigh organic fractions, such as catalyst residues, printed circuitboards and other electronic scrap:

-   -   1. Ecolyst® method (Umicore), described in German Patent DE 32        23 501C1/C2. This is a method for precipitation of, mainly, Rh        from liquid organic residues through the use of tellurium. An        organic mixture remains after separation of the precious metal        and must be disposed of (e.g., by combustion). The method        requires continuously transportable material.    -   2. Aquacat® method (Johnson Matthey). This method can be used to        process waste materials that contain carbon and organic        compounds and precious metals, in particular gold, silver,        platinum, and palladium waste from industrial production and the        watch and jewelry industry. The organic components are oxidized        with oxygen in supercritical water under pressure, with the        precious metal remaining behind as an oxidic residue. As before,        the material must be continuously transportable. In addition,        the method requires a pressurized reactor.

The aim is for an improved method possessing the following advantageousfeatures:

-   -   1) Batch-wise or continuous processing with the option of        continuous operation;    -   2) Efficient control of the thermal economy; and    -   3) High yield of the valuable substances.

The direct combustion (burning-off) of the organic fractions of preciousmetal-containing residues is already being utilized in various methods.Methods for the combustion of precious metal-containing sludges andmulti-element waste with subsequent leaching of the ash are described,for example, in German Patent DE 31 34 733 C2 and International PatentApplication Publication WO 99/37823. However, if the residues to betreated contain organic fractions that combust very easily and withgreat energy, there may be very intensive flame formation.

Siemens KWU developed a method for the treatment of private householdwaste, in which pyrolysis and high temperature combustion withutilization of the pyrolysis gases were combined (Ullmann's 6th ed.,CD-ROM-Release 2003, “Waste” Ref 320: K. J. Thomé-Kozmiensky: ThermischeAbfallbehandlung, EF-Verlag für Energie- and Umwelttechnik, Berlin1994). However, the crucial factor therein is the production of energyduring combustion (waste power plant).

Another possible method is the gasification of the organic fractions ofmetal-containing waste. German published patent application DE 33 29 042A1 relates to a method for the recycling of non-ferrous heavy metals andprecious metals from plastic-containing materials, in particular fromcoking products, in which the carbon is gasified under isothermalconditions with a separately generated gasification agent, such asH₂O/CO₂/O₂, whereby the temperature is regulated by the partialpressures. However, this method also is associated with major equipmentneeds and the presence of significant fractions amenable to gasificationat all times, since the gas ultimately serves to produce energy.

German Utility Model DE 94 20 410 U1 relates to a facility for a thermalrecycling method for metallic objects that are mixed with orcontaminated by organic substances, e.g., oil barrels, but also, on asmaller scale, precious metal-containing sweepings from jewelryworkshops or small operations of the jewelry industry. Carbonization inthe presence of a pyrolysis phase and an oxidation phase in acarbonization chamber is recommended in this context, whereby theoxidation proceeds with the introduction of a waste gas with acombustible oxygen content. According to German published patentapplication DE 35 18 725 A1, a similar facility serves for thermalvarnish removal—in this case, including the combustion of thecarbonization gases.

The two methods are not suitable for liquids with organic fractions thatcombust with great energy.

BRIEF SUMMARY OF THE INVENTION

Surprisingly, a simple process procedure allows both reduction of themajor plant resource need and realization of the improvement featuresmentioned above.

The invention relates to the burning-off of precious metal-containingmaterials with organic fractions that combust with great energy, in atleast two steps of which the first (A) includes a pyrolysis orcarbonization at reduced oxygen supply, and at least one additional step(B) comprises an oxidative combustion. A hot flame is not generatedduring the first process step. In the subsequent oxidative combustion ofthe pyrolysis residue, the flame and the emission of soot are limited.Preferably, soot emission is excluded.

The precious metal-containing materials with organic fractions are, inparticular, coals, solvents or plastic materials. Such materialsgenerally have a calorific value of 20 to 50 KJ/g, in particular about40 KJ/g, and are potentially explosive, if applicable. According to theinvention, steps A and B are carried out sequentially in a furnacechamber of an indirectly heated furnace. In this context, the pyrolysis,in German also termed carbonization, proceeds in an atmosphere with areduced oxygen content. For this purpose, the chamber is purged withprotective furnace gas, preferably nitrogen or argon. The oxygen contentis a maximum of 6 wt-%, preferably a maximum of 4 wt-%. The end of thepyrolysis is detected by a sensor, preferably a pressure sensor. At theend of process step A, the pyrolysis-treated material comprises noteasily volatized substances with a high carbon content. After thesensor-detected end of the pyrolysis, the atmosphere is changed bysupplying air or oxygen, and thus step B is initiated directly.Surprisingly, oxygen can be introduced into the furnace that is alreadyheated to 400 to 900° C., preferably 500 to 800° C., without anexplosion occurring. This process step saves substantial logisticresources, substantial energy use, and shortens the time needed.

The energy use can be reduced further by having two furnaces carry outsteps A and B in an alternating fashion and by these two furnaces eachbeing equipped with a single waste treatment step. Thus, the waste gasesof pyrolysis and the waste gases of combustion reach the waste gastreatment facility concurrently. This reduces the volume flux andtherefore the energy needs.

For a recycling furnace according to the invention, it is significantthat the burning-off chamber of the furnace be provided with a sensor,in order to be able to detect the end of the pyrolysis. It is alsosignificant that the pyrolysis furnace can be operated both under aprotective furnace gas as well as while supplying air or under an oxygenatmosphere, and is provided with a switching facility that can switchfrom the protective furnace gas filling of the furnace chamber to an airflow and/or oxygen flow. In this context, the switching must becontrolled as a function of the result obtained by the sensor.

In terms of the method, it is relevant that steps A and B are carriedout sequentially in a chamber, that the end of the pyrolysis isdetermined, and that a switch of atmospheres from protective furnacegas-containing atmosphere to air or oxygen atmosphere is effected afterthe end of the pyrolysis. This dispenses with a batch change and theensuing use of time and energy resources.

In a preferred embodiment, the furnace comprises a continuous conveyorfacility for liquids or pastes. For this purpose, during the pyrolysis,liquids or pastes are continuously conveyed into the pyrolysis chamberof the furnace, which is at a temperature of 300° C. to 700° C.,preferably 350° C. to 600° C. A risk of explosion is prevented in thiscontext by operating the furnace at a slight over-pressure.

Pastes get heated to the degree that they behave like liquids. Byconveying liquid substances or liquefied pastes through a conduit ofpipes, oxygen introduction is kept at such low levels that explosionlimits cannot be reached.

The precious metal fraction of the waste materials can vary widelydepending on its origin, e.g., from 0.01 to 60%. Metals other thanprecious metals can also be contained therein. Enrichment by thecontinuous conveying facility in the pyrolysis is particularly suitablefor waste materials with a precious metal fraction between 10 and 1000ppm (0.001 and 0.1 wt-%), preferably between 10 and 100 ppm (0.001 and0.01 wt-%), since considerable enrichment in a trough can be achievedalready during the pyrolysis by the continuous conveyance. Thecontinuous conveyance allows many times the liquid volume, relative tothe trough volume, to be pyrolyzed in a trough.

The furnace according to the invention is particularly important for therecycling of rhodium, platinum, palladium, gold, and iridium.

Conveyance of the liquids during the pyrolysis distributes thecarbonization gas generation evenly across the duration of the pyrolysisprocess. The carbonization gas generated during the pyrolysis reducesnatural gas consumption of the thermal post-combustion. Therefore, thethermal post-combustion can, on the one hand, be designed for lowervolume fluxes and, on the other hand, requires lower energy consumptiondue to the continuous supply of carbonization gas.

Preferably, the following measures that can be used individually or incombination with each other are undertaken in the method according tothe invention:

-   -   A. The method is advantageously carried out in a chamber        furnace.    -   B. Advantageously, two chambers for batch operation are present:        one can be used to carry out the pyrolysis followed by the        combustion, while the subsequent pyrolysis proceeds in the        second chamber already. When the second chamber is then switched        to combustion, the first chamber can already be supplied with        material for the pyrolysis.    -   C. It can have very advantageous effects to supply the material        for the pyrolysis step both slowly and continuously. Occasional        delays of boiling (detonations) that can occur upon batch-wise        addition can thus be avoided. The release of energy is more        even.    -   D. Naturally, the pyrolysis step proceeds under the virtual        exclusion of oxygen. It is suitable to purge the respective        chamber with protective furnace gas, preferably nitrogen, prior        to pyrolysis.    -   E. Usually, the temperature is controlled to a level of 100 to        1200° C., preferably 200 to 800° C., in step A, and a level of        500 to 1200° C., preferably from 600 to 800° C., in step B.    -   F. Liquid material is preferably supplied for pyrolysis step A        both slowly and continuously rather than batch-wise.    -   G. In step B, precious metal fractions and ash are usefully        received in capture troughs, which are arranged underneath the        combustion goods supply and/or pyrolysis goods supply.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown. In thedrawings:

FIG. 1 is a schematic diagram of the two chambers according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A suitable embodiment of a device of the invention is shownschematically in FIG. 1, which illustrates two combustion chambers withheating facility; capture troughs for the reception of precious metalfractions and ash, which are arranged underneath the combustion goodssupply and/or pyrolysis goods supply. For carrying out the methodaccording to the invention, two heatable combustion chambers 1, at leastone combustion/pyrolysis goods supply 3 each, and at least one capturetrough 2 each, arranged underneath the combustion/pyrolysis goodssupply, are provided.

The invention will now be described with reference to the followingspecific, non-limiting, possible examples.

Example 1

Five-hundred kg of different types of waste containing: precious metalfraction from 1 to 20 wt-%, with the elements Rh accounting for 0.001 to50 wt-%, preferably 0.1 to 20 wt-%, Pd accounting for 0.01 to 50 wt-%,preferably 0.1 to 20 wt-%, and organic fractions/solvent accounting for50 to 99.99 wt-%, preferably 80 to 99 wt-% of the precious metalfraction, are treated for 8 to 15 hours in a chamber furnace at anoxygen content of <4% and a temperature of 200 to 800° C.

Subsequently, the residue is burned-off while air is being supplied atan oxygen content of 14 to 16% and a temperature of 600 to 800° C.

Example 2

A chamber furnace is supplied with approx. 1100 kg of different types ofprecious metal-containing waste. For this purpose, 32 troughs with afilling volume of 60 liters each are introduced into the furnace.Fourteen of the troughs are filled with 30 kg each of a coal comprising0.1 wt-% palladium. Five troughs are filled with 20 kg platinum oxideeach, whereby the platinum oxide accounts for 80 wt-% and iscontaminated by 20 wt-% of a xylene-based solvent. Five troughs arefilled with 20 kg each of a paste from ceramic paint production, wherebythe paste comprises approx. 10 wt-% gold. Eight empty troughs are placedin the uppermost level of the furnace. The troughs are supported in abatch rack. Then the furnace is switched to pyrolysis operation. Forthis purpose, protective furnace gas is introduced into the furnaceuntil the oxygen content drops below 4 wt-%. The furnace, loaded at 200°C., is then heated to 600° C. over the course of 4 hours. The furnace isthen kept at 600° C. for two hours.

At this point in time, the pyrolysis in the loaded troughs is all butcomplete. Then, 500 liters organic liquid from a homogeneous catalystbased on rhodium in triphenylphosphine with added methylisobutylketoneis dosed into the empty troughs. The rhodium content is 10 ppm (0.001wt-%). The solution is continuously pumped into the 8 troughs, such thatit becomes distributed as evenly as possible. The pumping output ismaximally 200 l/hour and is regulated by the rate of utilization of thethermal post-combustion. For this purpose, the thermal post-combustionis provided with a temperature sensor, which reduces the pumping outputif the temperature rises above 1100° C. Consequently, the end of thepyrolysis is reached no earlier than after 2½ hours, or accordinglylater for higher calorific values according to the regulation by thethermal post-combustion. After the end of pumping, the temperature ofthe furnace is increased to 800° C. within approximately 30 minutes.

In the process, the over-pressure of 5 mbar established by theprotective furnace gas increases for a short period of time due to theproceeding pyrolysis. Once the over-pressure, detected with a pressuresensor, returns to the over-pressure of the nitrogen purge, this statusis maintained for 20 minutes at 800° C. If no further pressure increaseoccurs within this time of monitoring, atmospheric air is supplied tothe furnace chamber, and the oxidation phase is thus started withoutcooling of the chamber. A second furnace, connected to the same thermalpost-combustion as the furnace that has meanwhile been transferred tothe combustion phase, is then released for the start of a pyrolysis. Thethermal post-combustion is set to 1100° C. and is utilized moreefficiently by the combined operation.

The combined operation effects a more even release of carbonization gasquantities across the processes, A and B. By its nature, the thermalpost-combustion is designed for one furnace and is actually operatedwith two furnaces. This saves, for one, resources with respect todimensioning and, in particular, the energy costs from keeping thetemperature at 1,100° C. The natural gas consumption of the thermalpost-combustion is reduced further by the introduction of thecarbonization gases. This proceeds the more efficiently, the morehomogeneous the carbonization gas is introduced, which, according to theinvention, is achieved by coupling the post-combustion of one furnace inprocess B with a furnace that is being operated in process A.

According to the invention, the cooling of the furnace upon the switchfrom pyrolysis to combustion is saved, and thus, on the one hand, timeis saved and, on the other hand, energy for re-heating the furnace isalso saved. Moreover, the natural gas consumption for post-combustion inthe carbonization gas-free time of cooling is also saved.

The oxidation is then completed in known fashion, upon which the furnaceis cooled to 200° C., and the batch is removed.

Using the method according to the invention, various batches can beprepared without mutual mixing. For example, batches from variouscustomers can be processed in parallel, whereby the batches can differin nature.

The supply of liquids is also advantageous in that the carbonization gasquantity is made more even in order to save costs in the thermalpost-combustion. The liquids supplied at 600° C. can also be liquefiedpastes or suspensions. In the presence of oxygen, these liquids arepotentially explosive especially at these high temperatures. Theexplosion hazard is excluded according to the invention by keeping theoxygen content below 6 wt-%. Thus, potentially explosive substances aresurprisingly being supplied to a furnace at high temperatures.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A recycling furnace for processing potentially explosive preciousmetal-containing materials having organic fractions that combust withgreat energy, the furnace comprising: at least two combustion chambers,each of the at least two combustion chambers being provided withindirect heating, a protective furnace gas supply and an air or oxygengas supply and being attached to a single waste gas treatment facility,a switching facility for alternating operation of the at least twocombustion chambers between (A) pyrolysis or carbonization in anatmosphere comprising maximally 6 weight-% oxygen with gas from theprotective furnace gas supply and (B) oxidative combustion of theorganic fractions including carbon with oxygen from the air or oxygengas supply, and a sensor for determining an end of the pyrolysis orcarbonization in one of the at least two combustion chambers, whereinthe sensor controls the switching facility to supply the air or oxygento an interior of the one of the at least two combustion chambers inwhich the pyrolysis or carbonization is ending.
 2. The recycling furnaceaccording to claim 1, wherein the sensor is a pressure sensor or asubstance-sensitive sensor.
 3. The recycling furnace according to claim1, wherein each of the at least two combustion chambers comprises anexplosion-proof liquid supply facility feeding a hot chamber at atemperature of at least 400° C.
 4. The recycling furnace according toclaim 1, further comprising capture troughs, arranged underneath acombustion goods supply or a pyrolysis goods supply, for receivingprecious metal fractions and ash.
 5. The recycling furnace according toclaim 1, further comprising a continuous conveyer facility for dosingliquids or pastes into the one of the at least two combustion chambersduring pyrolysis.
 6. The recycling furnace according to claim 5, furthercomprising a temperature sensor for controlling the dosing of liquidsand pastes.
 7. The recycling furnace according to claim 1, furthercomprising at least one combustion goods supply for each of the at leasttwo combustion chambers and at least one capture trough for each of theat least two combustion chambers, the at least one capture trough beingarranged underneath the at least one combustion goods supply.